1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition and a piezoelectric ceramic element using the composition. More particularly, the present invention relates to a piezoelectric ceramic composition useful as a material for piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric ceramic resonators, and to a piezoelectric ceramic element using the composition.
2. Description of the Related Art
Conventionally, piezoelectric ceramic compositions comprising lead titanate zirconate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) as a main component are widely used for manufacturing piezoelectric elements such as piezoelectric ceramic filters. Recently, piezoelectric ceramic compositions comprising a bismuth layered compound such as CaBi4Ti4O15 as a main component have also been developed.
Pb, however, is suspected as a material which might cause environmental problems and affects human bodies. On the other hand, piezoelectric ceramics comprising bismuth layered compounds as the main components are not widely used in practice since the electromechanical coupling coefficients k33 are as small as less than 20%.
Therefore, it is an object of the present invention to provide a piezoelectric ceramic composition which does not contain Pb and which has an electromechanical coupling coefficient larger than that of a bismuth layered compound, and to provide a piezoelectric ceramic element using the composition.
A piezoelectric ceramic composition according to the present invention is characterized in that it comprises at least the elements Ag, Li, Nb, and O, and has an electromechanical coupling coefficient k33 of not less than about 20%.
It is also characterized in that it comprises a main component represented by the general formula (Ag1xe2x88x92xLix)(Nb1xe2x88x92yTay)O3, wherein 0.075 less than xc3x97 less than 0.40 and 0 less than y less than 0.20. In one embodiment, y=0 and the main component corresponds to (Ag1xe2x88x92xLix)NbO3. In another embodiment, y is greater than 0 and the main component corresponds to (Ag1xe2x88x92xLix)(Nb1xe2x88x92yTay)O3.
Furthermore, it is preferable that the composition comprises at least one of a manganese oxide and a silicon oxide as an auxiliary component. It is more preferable that the manganese oxide should be contained in an amount of not more than about 5 parts by weight based on 100 parts by weight of the main component, and that the silicon oxide should be contained in an amount of not more than about 5 parts by weight based on 100 parts by weight of the main component.
Furthermore, a piezoelectric ceramic element according to the present invention comprises a piezoelectric ceramic made from the piezoelectric ceramic composition according to the present invention, and an electrode formed on the piezoelectric ceramic.
In the general formula for the piezoelectric ceramic composition according to the present invention, a range of x less than 0.075 is not preferable since the transition temperature is decreased, across which a ferroelectric phase acting as a piezoelectric material turns into a paraelectric phase or an antiferroelectric phase which does not act as a piezoelectric material, with the result that there will be problems regarding the temperature stability of the elements composed of the piezoelectric ceramic composition. Also, when 0.40xe2x89xa6x, the resonance frequency constant will be smaller than 2,000 Hz/m, and polarization will not occur easily. When y is not less than 0.20, the transition temperature will be decreased, too. Accordingly, the ranges of 0.075xe2x89xa6x less than 0.40 and 0xe2x89xa6y less than 0.20 are required in the present invention.
Furthermore, in the present invention, the baking temperature can be decreased by adding a manganese oxide or a silicon oxide to the main component. It is to be noted that it is preferable that the manganese oxide should be contained in an amount of not more than about 5 parts by weight based on 100 parts by weight of the main component, and that the silicon oxide should be contained in an amount of not more than about 5 parts by weight based on 100 parts by weight of the main component, so as not to degrade the properties that are achieved when no such materials are added.
Furthermore, the piezoelectric ceramic composition according to the present invention may be a solid solution, a mixture, a polycrystal or a single crystal.
The above-described objects, other objects, features, and advantages of the present invention will become more apparent from the following detailed explanations of the embodiments when taken in conjunction with the accompanying drawings, tables, and examples.